For cathode-ray tube control, circuits having high output voltages are necessary as the control voltages of the tubes can go beyond 200 V. The control circuits of such tubes are generally amplifiers. However, for monochromatic monitors, it is possible to use simple logic gates.
For a clearer explanation of the problems encountered, reference may be made to FIG. 1 which will be well known to those skilled in the art and will provide a schematic view of an amplifier 1 controlling the emission cathode of a cathode-ray tube 2. An electrical model of the cathode of the cathode-ray tube is provided by a capacitor 3. The resistors R1 and R2 are used to define a factor of amplification equal to R1/R2. The amplifier 1 is supplied with two supply voltages, VCC (for example 12 V) and VDD (for example 240 V).
As is known to those skilled in the art, the amplifier 1 has an input stage 4, an amplifier stage 5 and an output stage 6. The passage from the low voltage (12 V) to the high voltage (240 V) is done in the output stage to reduce the consumption and heating of the amplifier 1. Naturally, there are other variants and the number of stages of the amplifier 1 may vary.
Conventionally, the cathode of the tube 2 does not react above a voltage of about 200 V and gets saturated below a voltage of about 50 V. Thus, a pixel that is illuminated to the maximum intensity with an extinguished pixel on each side is electrically expressed at the input of the cathode by a voltage square-wave of 50 to 200 V. If, furthermore, we look at a screen with 640 lines of 1024 pixels scanned at 100 Hz, a voltage square-wave time of 15 ns is obtained. The slew rate at output of the amplifier 1 should therefore be at least equal to 10 V/ns. Furthermore, the value of the capacitor 3 is conventionally 12 pF, which necessitates the production of a current of at least 120 mA.
A reliable embodiment of cathode driving circuits uses very costly hybrid technologies. Monolithic embodiments have been made in bipolar technologies to reduce the cost of the circuits. However, the bipolar technologies have fairly high power consumption and suffer from problems of switch-over speed with the PNP transistors. Furthermore, the bipolar transistors have switching times that greatly depend on the current flowing through them.